Not Applicable
Not Applicable
1. Field of the Invention
This invention relates to electronic article surveillance (EAS) systems, and markers and labels for use therein, and more particularly to a new bias configuration for magnetomechanical and magnetoacoustic EAS markers.
2. Description of the Related Art
U.S. Pat. No. 4,510,489, the ""489 patent, discloses an EAS marker made of an elongated strip of magnetostrictive ferromagnetic material disposed adjacent to a ferromagnetic element that, when magnetized, magnetically biases the strip and arms it to resonate mechanically at a preselected resonant frequency. The marker resonates when subjected to an interrogation field at a frequency at or near the marker""s resonant frequency. The response of the marker at the marker""s resonant frequency can be detected by EAS receiving equipment, thus providing an electronic marker for use in EAS systems. As used herein, the term xe2x80x9cmarkerxe2x80x9d refers to markers, labels, and tags used in EAS systems.
Referring to FIG. 1, the marker of the ""489 patent is constructed of a resonator, an elongated ductile strip of magnetostrictive ferromagnetic material 18, disposed adjacent a ferromagnetic element 44. Element 44 is a high coercivity biasing magnet that, when magnetized, is capable of applying a DC magnetic field to resonator 18 such that resonator 18 is provided with a single pair of magnetic poles, each of the poles being at opposite extremes of the long dimension of resonator 18. Resonator 18 is placed within the hollow recess or cavity 60 of housing 62 with bias 44 held in a parallel adjacent plane so that bias 44 does not cause mechanical interference with the vibration of resonator 18. Because resonator 18 must vibrate freely within cavity 60 and bias 44 is maintained in a parallel adjacent plane, the marker has a required minimum thickness to accommodate the adjacent parallel planes and permit free vibration of resonator 18.
Due to the close proximity of bias 44 and resonator 18, a substantial magnetic attraction exists between the resonator and the bias. The magnetic attraction causes the resonator to be pulled within its cavity toward the bias, and into a bias field region that may be slightly different than the desired bias field disposed near the center of the cavity. The magnetic attraction results in a significant loss of signal amplitude from mechanical friction between the resonator and its cavity, and from the bias instability due to the position of the resonator. To overcome the magnetic xe2x80x9cclampingxe2x80x9d or damping of the free vibrations of the resonator, the resonator can be annealed with a transverse curl to minimize the magnetic attraction. As a result of the curled resonator, the marker cavity must be made deeper for the resonator to vibrate freely. An even thicker marker results from the deeper cavity required to accommodate the curled resonator. U.S. Pat. No. 5,568,125 discloses a process for making a resonator with a transverse curl.
There are presently EAS marker applications in which a flat marker is desired. A flat EAS marker is defined herein as an EAS marker of lower minimum thickness than is required to accommodate a bias and a resonator that are maintained in parallel adjacent planes as illustrated in FIG. 1. A flat marker can provide a larger surface area for the attachment of indicia, and may be more bendable.
Referring to FIGS. 2 and 3, U.S. Pat. No. 4,727,360, the ""360 patent, discloses a flat marker in which the resonator 48 and bias 50 are configured in a side-by-side relationship separated by a preselected distance xe2x80x9cdxe2x80x9d, and disposed within the same plane as shown in FIG. 3. Unlike the marker disclosed in the ""489 patent and described above, the marker of the ""360 patent is a frequency-dividing marker. The frequency dividing marker of the ""360 patent has a resonant frequency xe2x80x9cfxe2x80x9d, which when subjected to an interrogation frequency of xe2x80x9c2fxe2x80x9d responds with a subharmonic of the frequency xe2x80x9c2fxe2x80x9d.
Referring to FIGS. 4 and 5, U.S. Pat. No. 5,414,412, the ""412 patent, discloses a frequency-dividing marker that is an improvement to the marker disclosed in the ""360 patent. The marker disclosed in the ""412 patent includes a tripole bias magnet 54 disposed adjacent resonator 52 and on the opposite side from bias 51, all of which are disposed in the same plane, to achieve improved frequency-dividing performance.
As discussed above, the markers of the ""360 and ""412 patents are frequency-dividing markers that do not operate in the same manner as the marker disclosed in the ""489 patent. However, if a similar bias orientation, one that is positioned to the side of the resonator and in the same plane, is used in a marker of the type disclosed in the ""489 patent to produce a flat magnetomechanical label, problems result. Having a single bias disposed to the side of the resonator results in a relatively lower magnetic coupling and requires an increased minimum amount of bias material to properly bias the resonator. Magnetic clamping thus results between the resonator and the larger bias. As described above, the magnetic clamping is due to magnetic attraction between the bias and the resonator that results in a xe2x80x9cclampingxe2x80x9d or damping of the free vibrations of the resonator thereby reducing the amplitude of the resonator""s response at its preselected resonant frequency. In addition, a single bias disposed to the side of the resonator of sufficient size to properly bias the resonator results in a thick and/or wide bias that tends to demagnetize itself. The demagnetizing effect of the bias causes deterioration in the stability of the label over time.
The present invention is a magnetomechanical electronic article surveillance marker that has a magnetostrictive resonator made of an amorphous magnetic material. The resonator is sufficiently elongated to have a longitudinal axis. A pair of bias magnets, also each having a longitudinal axis, are disposed on opposite sides and adjacent the resonator to bias the resonator with a magnetic field of a preselected field strength. The pair of bias magnets and the resonator can be relatively equal in length, and are positioned in a housing and maintained substantially parallel and coplanar with each other.
The bias magnets are magnetized along their lengths each having a north and a south magnetic pole disposed at opposite ends of each of the bias magnets. The bias magnets are disposed adjacent the resonator so the north pole and the south pole of each bias magnet are adjacent each other and adjacent opposite ends of the resonator.
In one embodiment, the bias magnets are about 6 mils thick by about 3-mm wide by about 3.7-cm long with a separation between the pair of bias magnets of about 1.15-cm. The resonator disposed between the bias magnets is then about 1 mil thick by about 6-mm wide by about 3.8-cm long. Multiple resonators can be disposed between the bias magnets in an alternate embodiment.
In one embodiment, the preselected bias magnetic field strength is about 6.5 Orested (Oe) and the resonator is adapted to resonate at a frequency of about 58 kHz. The bias magnets can be made of a semihard or hard magnetic material.
The bias magnets disposed within the housing can be adjustable in position relative to the resonator, which changes the bias spacing to compensate for measurable variances in preselected magnetic properties of the amorphous magnetic material and the bias magnets, and/or to adjust the resonant frequency of the marker. The housing can include a first cavity sized to capture the resonator so that said resonator is free to resonate, and a second and a third cavity on opposite sides of the first cavity to retain one each of the bias magnets in a preselected position. Alternately, the housing may have one cavity or another configuration so that the resonator is free to vibrate and the bias magnets are maintained in a preselected position.
In an alternate embodiment, the lengths of the bias magnets relative to the resonator can be varied to compensate for measurable variances in preselected magnetic properties of the amorphous magnetic material and the bias magnets, and/or to adjust the resonant frequency of the marker.
Objectives, advantages, and applications of the present invention will be made apparent by the following detailed description of the preferred embodiments of the invention.